Wires of metals such as tungsten, copper, iron, molybdenum, and stainless steel are produced by drawing the metals through diamond dies. Single crystal diamond dies are difficult to fabricate, tend to chip easily, easily cleave, and often fail catastrophically because of the extreme pressures involved during wire drawing.
With reference to single crystal wire dies, it is reported in Properties and Applications of Diamond, Wilks et al, Butterworth-Heinemann Ltd. 1991, pages 505-507: "The best choice of [crystallographic] direction is not too obvious because as the wire passes through the die its circumference is abrading the diamond on a whole 360.degree. range of planes, and the rates of wear on these planes will be somewhat different. Hence, the originally circular hole will not only grow larger but will loose its shape. However, &lt;110&gt; directions offer the advantage that the wire is abrading the sides of the hole with {001} and {011} orientations in abrasion resistant directions."
Diamond dies which avoid some of the problems attendant with natural diamonds of poorer quality comprise microporous masses compacted from tiny crystals of natural or synthesized diamonds or from crystals of diamond. The deficiencies of such polycrystalline hard masses, as indicated in U.S. Pat. No. 4,016,736, are due to the presence of microvoids/pores and soft inclusions. These voids and inclusions can be more than 10 microns in diameter. The improvement of the patent utilizes a metal cemented carbide jacket as a source of flowable metal which fills the voids resulting in an improved wire die.
European Patent Application 0 494 799 A1 describes a polycrystalline CVD diamond layer having a hole formed therethrough and mounted in a support. As set forth in column 2, lines 26-30, "The relatively random distribution of crystal orientations in the CVD diamond ensures more even wear during use of the insert." As set forth in column 3, lines 50-54, "The orientation of the diamond in the polycrystalline CVD diamond layer 10 may be such that most of the crystallites have a (111) crystallographic axis in the plane, i.e. parallel to the surfaces 14, 16, of the layer 10."
Other crystal orientations for CVD films are known. U.S. Pat. No. 5,110,579 to Anthony et al describes a transparent polycrystalline diamond film as illustrated in FIG. 3A, substantially transparent columns of diamond crystals having a &lt;110&gt; orientation perpendicular to the base.
Because of its high purity and uniform consistency, CVD diamond may be desirably used as compared to the more readily available and poor quality natural diamond. Because CVD diamond can be produced without attendant voids, it is often more desirable than polycrystalline diamond produced by high temperature and high pressure processes. However, further improvements in the structure of CVD wire drawing dies are desirable. Particularly, improvements in grain structure of CVD diamond wire die which tend to enhance wear and uniformity of wear are particularly desirable.